Method of impregnating porous refractory bodies with inorganic chromium compound

ABSTRACT

A new ceramic treatment process and product and, more particularly, a process for forming and treating and shaping and treating of porous skeletal bodies to produce hardened dimensionally stable end products at relatively low temperature and the resulting materials produced thereby which comprises the steps of treating a porous skeletal body by impregnating the interstices of the porous body with a comound of a metal capable of being converted to an oxide in situ at relatively low temperatures, heating the body so impregnated at temperature well below normal vitrification to a temperature of at least 600*F. and for a period of time sufficient to convert the compound impregnated therein to an oxide and repeating the impregnation and heating steps until the desired degree of hardness is obtained.

United States Patent 1 Church et a1.

METHOD OF IMPREGNATING POROUS REFRACTORY BODIES WITH INORGANIC CHROMIUM COMPOUND Inventors: Peter K. Church, Cascade; Oliver .1. Knutson, Colorado Springs, both of Colo.

Kaman Sciences Corporation, Colorado Springs, Colo.

Filed: Dec. 28, 1967 Appl. No.2 694,303

Related US. Application Data Continuation-impart of Ser. No. 642,704, June 1, 1967, abandoned.

Assignee:

US. Cl 264/60, 106/59, 106/66, 106/67, 1l7/l07.2, 264/62, 264/66 Int. Cl. C04b 41/24, C04b 41/06 Field of Search 51/307, 295, 309; 106/55-67; 264/56, 60, 66, 62; 117/61, 107.2

References Cited UNITED STATES PATENTS 2,537,012 l/l951 Austin et al. 106/62 2,537,013 1/1951 Austin et a1. 106/62 2,798,809 7/1957 Goetzel et al 51/295 2,828,225 3/1958 Goetzel et al....... 106/57 3,050,409 8/1962 Bayer 106/55 3,255,027 6/1966 Talsma 106/62 Primary Examiner-Donald .1. Arnold Attorney, Agent, or FirmAnderson, Spangler,

Wymore & Klaas [57] ABSTRACT A new ceramic treatment process and product and, more particularly, a process for forming and treating and shaping and treating of porous skeletal bodies to produce hardened dimensionally stable end products at relatively low temperature and the resulting materials produced thereby which comprises the steps of treating a porous skeletal body by impregnating the interstices of the porous body with a comound of a metal capable of being converted to an oxide in situ at relatively low temperatures, heating the body so impregnated at temperature well below normal vitrification to a temperature of at least 600F. and for a period of time sufficient to convert the compound impregnated therein to an oxide and repeating the impregnation and heating steps until the desired degree of hardness is obtained.

26 Claims, 13 Drawing Figures PATENTED 3,789,096 SHEET 1 BF 3 FIGT4 INVENTORS PETER K. CHURCH OLIVER J. KNUTSON ATTORNEYS PATENTEBJANZQ I974 sum 2 OF 3 FIG. 7

FIG.

FIG. 5

FIG. 10

FIG.

FIG.

INVENTORS CH U R C H PETER K.

FIG. OLIVER J. KNU'TSON ATTORNEYS PATEHTEUJANZQIQH SHEET 3 BF 3 FIG. 12

FIG. 13

INVENTORS PETER K. CHU RC H OLIVER J. KNUTSQN ATTORNEYS 1 METHOD OF IMPREGNATING POROUS REFRACTORY BODIES WITH INORGANIC CHROMIUM COMPOUND ln applicants previously filed applicationfS er. No. 642,704filed June 1, 1967 and now abandoned in favor of continuation application Ser. No. 63,998, filed June 18, 1970, now U.S. Pat. No. 3,734,767 issued May 22, 1973, of which the present application is a continuation-in-part, applicants disclosed the process of treating underfired porous partially vitrified relatively soft refractory ceramic which comprises the steps of shaping an underfired partially vitrified relatively soft refractory ceramic into a predetermined shape, im-

pregnating the shaped ceramic with phosphoric acid vert the impregnant to an oxide to harden the cerar nic.

Ceramic materials normally undergo substafiti al dimensional changes during the usual firing or vitrification steps. Thus, it has heretofore been extremely difficult to produce precision parts or intricate shapes from ceramics. Precision parts had to be shaped slightly oversize before firing. After firing, the parts required further machining with diamond cutting wheels or by using lapping methods. Many intricate shapes were just not available since thin sections of parts would crack during firing.

In accordance with the invention, it has been found that underfired or so-called machinable grade refractory ceramics can be shaped while in the relatively soft state and then impregnated and heat treated to produce a ceramic having all the desirable characteristics of a vitrified ceramic without the usual change in dimensions. The process of the instant invention appears to be useful in the treatment of such refractory ceramic materials as the oxides of aluminum, beryllium, zirconium, titanium, magnesium and the like. These materials in the commercially available machinable grade are quite soft and easily broken. Also, in the soft state, they can be readily cut with carbide cutting tools, drilled, filed, sanded and otherwise formed to practically any desired shape. One such aluminum and beryllium oxide material is available from Coors Porcelain Company of Golden, Colo. When the machinable ceramics are treated by the method of this invention, they become very hard, approximating highly vitrified ceramic and, in addition, will retain the original machined and pretreated dimensions. The treated material becomes so hard that the only practical method to do further machining is with diamond cutting wheels or by using lapping techniques.

The commercial value of the instant invention is.

readily seen when it is recognized that close tolerances on many intricate vitrified ceramic parts can only be obtained by machining with diamond cutting methods after firing. This is the case since there is considerable shrinkage which occurs during the firing. Also, there are many desired shapes which cannot be economically cast or molded using a high firing process. In addition, it is often not feasible to construct molding dies for small quantities of a particular part. The method of the present invention in contrast thereto permits easy machining of parts to exact tolerances and then hardening the part without change in original dimensions.

It has now been found that the hardening process may be equally applied to the hardening of nonsintered bodies. It has been found that the base refractory material can be prepared in a powdered form (such as ball-milled aluminum oxide) and simply pressed, molded, slip cast, extruded, or otherwise processed so that the base oxide particles are packed into close proximity to provide a porous body. The hardening of the non-sintered bodies is essentially the same method as applied to the porous, partially sintered materials. The hardening is accomplished by impregnating the porous body with a metal compound, which may be in solution, which compound is capable of being converted to the metal oxide in situ at a temperature below sintering temperature in the range of from about 600F. to about 1,500F. and heating the body to convert the compound to its oxide. The impregnation and cure cycle must be repeated at least for two cycles to provide any usable hardening. It has further been found that other finely divided materials, such as a powdered metal, oxide mixtures and the like will serve as the base material which may also contain additives such as glass or metal fibers or abrasive grains to provide special characteristics in the finished product.

It is, therefore, the principal object of this invention to provide an improved low temperature process for the forming and treating and shaping and treating of relatively soft porous bodies which avoids one or more of the disadvantages of prior art methods of producing close tolerance hardened shaped parts.

A further object of the present invention is to provide an improved low temperature process of producing hardened articles of manufacture of predetermined shapes, of predetermined characteristics and of predetermined dimensions.

Another object is to provide an improved low temperature method of producing articles of manufacture in close tolerance shapes of selected hardness, porosity and surface characteristics.

A still further object of the invention is to provide an V improved process for the production of ceramic bear ings capable of use with or without lubricants under un favorable conditions.

A further object of the invention is to provide an improved process for the application of a refractory oxide coating to a substrate and/ or the hardening of the oxide coating applied thereto.

A further object of the invention is to provide an improved low temperature process for the production of improved abrasive or polishing stones and grinding wheels which may include abrasive grain additives.

A further object of the invention is to provide a process for the production of a refractory ceramic oxide material having a negative temperature coefficient of electrical and heat conduction.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken with the I magnification of a pressed body of Alcoa T6l(-325 mesh) alumina ball milled 48 hours and chemically hardened;

FIG. 6 is a 200 metallographic photograph of a pressed body of Alcoa T6l(325 mesh) alumina ball milled 96 hours and chemically hardened;

FIG. 7 is a 200X metallographic photograph of a pressed body of beryllium oxide powder which has been chemically hardened;

FIG. 8 is a 200X metallographic photograph of a pressed body of chromium oxide powder which has been chemically hardened;

FIG. 9 is a 200X metallographic photograph of a pressed body of Alcoa T-6 1 (-325 mesh) ball milled 96 hours with aluminum fibers added and chemically hardened;

FIG. 10 is a 200X metallographic photograph of a refractory oxide painted on a metal substrate and chemically hardened;

FIG. 11 is a photograph of a commercial sintered grinding wheel side by side with a grinding wheel made by the process of this invention;

FIG. 12 is a 300 metallographic photograph of a cross section through a commercial plasma sprayed chromia coating prior to treatment according to this invention; and,

FIG. 13 is a 300X photograph of the cross section of FIG. 12 after treatment.

This invention is directed to a process and product involving new types of materials that are formed by multiple chemical impregnations of a relatively soft porous body of finely divided refractory oxide base materials, each followed by a low temperature cure to convert the impregnant to an oxide. The resulting ceramic structure formed in this manner has been shown to exhibit extreme hardness, a high compressive strength and a dimensionally stable material over a wide temperature range. In addition, a number of these new ceramic materials show an inherently small coefficient of friction coupled with a very low wear rate characteristic.

Parts can be economically fabricated of this new material in a wide variety of intricate shapes and sizes. This is most easily accomplished by providing the base refractory material in a powdered form and packing the powder articles into close proximity by suitable means to provide a porous body of predetermined shape. The shaped pieces are then repeatedly chemically treated and cured at a temperature substantially below that used for normal ceramic vitrification.

One of the unique features of this chemical treatment and hardening method is that virtually no change occurs in the original dimensions of the shaped part during the hardening process. Therefore, expensive diamond machining of the finished hardened part is eliminated.

Many of these new ceramic materials will withstand repeated water quenching from 1,000F. as well as prolonged exposure to temperature extremes of 2,000F. to 300F. Mohs scale hardness is in excess of 9, normally being about equal to that of silicon carbide. Rockwell hardness can be as high as A- to A-90, with associated compressive strengths in excess of 125,000 psi. I

In addition to their use for the manufacture of precision parts, many of these ceramics exhibit excellent characteristics for low friction and low wear rate bearing and seal applications; in particular, journal bearings, thrust bearings and sliding type bearings and seals. When used in this manner, lubrication may be by means of a wide variety of conventional and nonconventional lubricants. Among those successfully tested to date include: tap water, sea water, alcohol, kerosene, polyethylene glycol, trichlorethylene, lubricating oils, silicone fluids and liquid metals. Solid lubricants have been used with good results at temperatures up to about 2,000F. In addition, lightly loaded bearings have been operated for limited periods at high speed without lubrication.

Life tests of sleeve-type bearings have been and still are currently in process. However, to date wear has been too low to obtain quantitative data, even after many months time. Rub-shoe type wear rate tests have consequently been conducted and have shown exceptionally low wear rate characteristics. For example, a ceramic shoe of this invention riding on a ceramic wheel of the same material exhibited many times less wear than a bearing bronze shoe riding against a steel wheel using oil as the lubricating media. Also, unlike a conventional bronze-steel bearing combination, very heavy loads can be applied to many of the ceramic-toceramic material bearings without their showing any tendency toward galling, even when running with such poor lubricants as alcohol or water.

A special variation in treatment of this invention has also been found that will produce a honing or finishing material that appears to be superior in several respects to both natural and artifically produced grinding stones. For example, one such ceramic will remove metal far more rapidly than will an Arkansas stone, while at the same time producing a finer and more highly polished finish. Another ceramic material of this invention displays a wide variation in electrical and heat conduction with relatively small changes in temperature.

The basic method employed for producing the new ceramic materials consists of chemically impregnating a porous, refractory oxide structure followed by a low temperature cure. The porous refractory acts as the skeletal framework around which the final ceramic structure is formed.

The simplest chemical hardening method consists of impregnating the shaped porous body with a solution of chromic acid. The thoroughly impregnated material is then cured in an oven with the final temperature reaching at least 600 1,000F or higher. The impregnation and curing cycle is repeated at least once. With a suitable refractory base material, this simple acid treatment will produce a hard ceramic body having numerous uses.

The finely divided base material may be mixed with a binder, such as kaolin and the like, before shaping or the impregnant may serve as the binder after the first cure. This also may be accomplished by impregnation of the porous structure with a water solution of a soluble metal compound convertible toan oxide and subsequently converting same to the oxide by simply elevating the temperature to the required conversion point. The metal compound is selected so that the oxide conversion will normally take place at a temperature less than about 1,500F.

As mentioned previously, the ceramic material is built around a porous refractory base material that functions as the skeletal structure. The types of such materials that are suitable for use in the present invention include various grades of alumina, titania, beryllia, magnesia, magnesium silicate and stabilized zirconia. Some materials were obtained from the manufacturer in an underfired or machinable" form. Int his condition, these materials were normally found to be soft enough to allow machining by conventional means, and exhibited a relatively high effective porosity (10 to percent) to allow for subsequent chemical treatment by the process of this invention. Table 1 lists the major type designation, manufacturer, and indicated hardness, porosity and fabrication method for each of the skeletal refractory materials tested.

10 The addresses of the manufacturers referred to in Table l are as follows: American Lava Corp., Chattanooga, Tenn.; Amerisil, lnc., Hillside, N.J.; Coors, Golden, Colo.; and, Du-Co Ceramics, Saxonburg, Pa.

TABLE I UNDERFIRED, POROUS REFRACTORY BASE MATERIALS Manufacturer's Base Type Maior Material Designation Manufacturer Oxide Alumina AHP-99 Coors 99% Al2O3 Alumina AP-99-L3 Coors 99% Al2O Alumina AP-99-ll Coors 99% A| O Alumina AP-99-l2 Coors 99% Al2O Alumina AP-99-Ll Coors 99% M 0 Alumina AP-99-L2 Coors 99% M 0 Alumina AP-99C-Ll Coors 99% Al2O Alumina AP99C-l2 Coors 99% M 0 Alumina AP-99C-L3 Coors 99% Al O Alumina AlSiMag 614 Am.Lava Corp. 96% Al2O Alumina AlSiMag 39s AmJ-dtlq Cor 96% A1565 Alumina AlSiMag 548 Am.Lava Corp. 99.8% M 0 Beryllia BP-96-il Coors 96% BeO Magnesia l87E4 Du-Co Ceramics 89% M90 Magnesia l87E77 Du-Co Ceramics 96% M90 Magnesium AlSiMag 222 Am.Lava Corp. MgO.SiO2 Silicate SillCO 3 Porosity Amersil, Inc. 99% SiO Zirconia l72H20 Du-Ca Ceramics ZrO Titania AlSiMag I92 Am.Lava Corp. 98% TiO (Underfirecl) Alumina AP-995-L3 Coors 99.5% Al O Alumi na AP-997-L3 cddrs' 99.7% M 6};

Alumina AP-94-ll Coors 94% Al O Other Sintering Effective Mahs Oxides Terrp. PorosiD Hardness Remarks 0.5% SiOz 2670 F. 45.7% 2-3 lsostalic 0.2% CaO Pressed 0.2% M90 2570 F. 42.4% H Extruded 17oo F. 0-1 Extruded 21ao F. 1 Extruded 2642 F Extruded I 2670 F 5-6 Extruded 2642 F. 4-5 Cast 2T3D F Cast 2 57o F. Cast SiO 2000 F l-Z ordered green, M90 fired for C00 20 min. at 2000F u an *L49ed FQ 17oo F. 1-2 Extruded SE0 20oo F. 1-2

SiOz 20oo F l-2 2-3 Hot Pressed 5% C00 1-2 Made from ZCA Type F'Coarse Grain Zirconiu- (CaO stabilized) SE0 20oo F. 2-3 Ordered Green MgO fired 20 min. CaO at ZOOO F 2570 F Extruded T 7 570 ti fist 3 .75% SiOg 33 l% 2-3 Extruded 0.9% C00 0.75% M 0 I700 F. 0.5% ZrO A A w 7 A. 7 8

TABLTE I (CONTINUEDY Manufacturer's Bose Type Moior Other Sintering Effective Mohs Material Designation Manufacturer Oxide Oxides Terrp Porosity Hardness Remarks Alumina AP-94 i5 Coors W 94% M 3.75% si o 21so F. 33.0% 2-3 Extruded 0.9% C00 0.75% M 0 0.5% ZrOz 0.1% F 0 Alumina AP-94-l2 Coors 94% A|2O 2130 F 44.l% 2-3 isostatic (isostatic) Pressed Alumina AP-85-Il Coors 85% Al203 10% SiO2 l700 F 33.4% 2-3 Extruded 2.75% M90 1 .25% C00 0.75% 500 0.25% Fe2O Alumina AlSiMag 6l4 Am.Lcvo Corp. 96% M 2000 F 6-7 Too hard for (underfired) easy machining These materials are fabricated by one or more of several commercially used methods such as powder pressing, extrusion, isostatic forming or slip casting. The important factor, however, is that the formed or pressed oxide be only partially sintered since optimum sintering will result in a dense body with insufficient porosity to be usable in the chemical treatment method of this invention.

In addition to the alumina, beryllia, magnesia, titania and zirconia materials, it is anticipated that many of the other partially sintered refractory oxides would make applicable skeletal structures for the improved ceramic material. Among these would be the oxides of Barium, Calcium, Cerium, Chromium, Cobalt, Gallium, Hafnium, Lanthanum, Manganese, Nickel, Niobium, Tantalum, Thorium, Tin, Uranium, Vanadium, Yttrium and Zinc. Also, many of the complex-refractory oxides should be suitable base materials. Of the complexrefractories, only the magnesium silicate has been tested to date. Other complex-refractories that may be suitable if produced in a porous, partially sintered (underfired) form are Aluminum silicate, Aluminum titanate, Barium aluminate, Barium silicate, Barium zirconate, Beryllium aluminate, Beryllium silicate, Beryllium titanate, Beryllium zirconate, Calcium chromite, Calcium phosphate, Calcium silicate, Calcium titanate, Calcium zirconate, Cobalt aluminate, Magnesium aluminate, Magnesium chromite, Magnesium ferrite, Magnesium lanthanate, Magnesium silicate, Magnesium titanate, Magnesium zirconate, Magnesium zirconium silicate, Nickel aluminate, Potassium aluminum silicate, Strontium aluminate, Strontium phosphate,

Strontium zirconate, Thorium zirconate, Zinc aluminate, Zinc zirconium silicate and Zirconium silicate.

The novel process according to the invention is particularly adapted to the treating of porous, partially vitrifled refractory ceramics such as the oxides of Aluminum, Barium, Beryllium, Calcium, Cerium, Chromium, Cobalt, Gallium, Hafnium, Lanthanum, Magnesium, Manganese, Nickel, Niobium, Tantalum, Thorium, Tin, Titanium, Uranium, Vanadium, Yttrium, Zinc and Zirconium and mixtures thereof. The oxides may be substantially pure or may contain or have amounts of impurities or additives, such as an oxide of a metal other than that of the body such as Cadmium, Chromium, Cobalt, Copper, Iron, Magnesium, Manganese, Nickel,

Titanium and the like and/or other salts of such metals which ultimately will convert to oxides at least during the final curing step. The process of this invention also contemplates the addition of amounts of additives such as a salt of a metal other than that of the body and convertible to an oxide such as the acetates, chlorides, nitrates and oxalates of Aluminum, Beryllium, Cadmium, Calcium, Cerium, Chromium, Cobalt, Copper, Iron, Lanthanum, Lithium, Magnesium, Molybdenum, Nickel, Strontium, Thorium, Tin, Tungsten, Zinc and Zirconium which are added to the ceramic during treatment.

The process of this invention may comprise the forming of a partially sintered untreated ceramic into a predetermined shape of the forming thereof from a powder and a binder. It will be understood that, while precast machinable stock may be used, it is possible to precast to intricate shapes and prefire to an underfired condition before the ceramic is subjected to applicants process. The ceramic, either stock orformed, is usually quite porous. The simplest method of chemically hardening the porous refractory structure is with a phosphoric acid treatment; however, this precludes multiple treatments as the reaction seems to go to completion in one treatment. The ceramic is impregnated with a concentrated phosphoric acid solution, usually of percent concentration. The ceramic can be evacuated in a vacuum before immersion in the acid to hasten the impregnation or, as has been found to be particularly effective, the ceramic can be heated to from about 300 to about 6 00F. and then immersed in the phosphoric acid solution. The heating causes a vacuum to be produced within the voids of the ceramic and the phosphoric acid is drawn all through the ceramic upon immersion. While a considerably longer time is required, the ceramic also can be just immersed in the phosphoric acid solution for a length of time sufficient for complete impregnation. Greater uniformity is achieved by using the vacuum or heating impregnation techniques. When the part is thoroughly impregnated with phosphoric acid, it is removed from the solution, excess phosphoric acid on the surface is drained or wiped off.

Next, Applicants novel process comprises the controlled heat curing of the phosphoric acid impregnated ceramic. The heating cycle is usually started around F. and ends at about at least about 900F. The ceramic pieces are preferably placed in powdered asbestos, and the like, to minimize shock during the heating and cooling cycle. The powdered asbestos also serves to absorb liquid driven out of the ceramic as the temperature is raised. The excess liquid, if not absorbed, would be likely to craze the surface of the ceramic.

As pointed out, one of the unique features of the method of the invention is that virtually no dimensional changes occur in the machined piece during the hardening process. Therefore, expensive diamond-type machining of a hardened part is eliminated.

The property of physical hardness has been used as the primary means of determining effects of varying the measurements for various materials which have been given a simple phosphoric acid treatment.

acid phosphoric treatment which may be given if desired usually produces an even harder end product.

Where the ceramic material is impregnated with a high concentration of phosphoric acid and heat treated, a good bearing material is produced and two pieces of this same material will slide against one another with a low coefficient of friction. After such pieces are worn in for a short while, a shiny surface film is produced which remains shiny even at elevated temperatures. Where the more concentrated phosphoric acid is used, the resulting product is more dense with smaller unfilled pores. Where a relatively pure ceramic oxide is treated, the addition thereto of another oxide during treatment substantially increases the hardness of the finished product. While it is not completely known what occurs in the treating process, the pores of the underfired ceramic are believed to be filled or partially TABLE II HARDNESS MEASUREMENWSIMPLE ACID TREATMENT Base Type M: ior H3 P04 Mobs Rockwell Sanple Material Designation Manufacturer Oxide lnpregnatian Hardness Hardness Remarks 2l E Alumina AP-85-Il Coors 85% Al2O3 85% 8-9 A-66.5

22E Alumina AP-94-ll Coors 94% Al2O 85% 6-7 A-69.5

23E Alumina AP-94-l2 Coors 94% Al O 85% 6-7 A-7l .0

24E Alumina AP-94-l2 Coors 94% Al2O 85% 6-7 A-57.5

(isostatic) 25E Alumina AP-99-L3 Coors 99% A|2O 85% 8-9 A-70.5 Y

205 Alumina AHP-99 Coors 99% Al O 85% 6-7 A-52.5

A7 Alumina AlSiMag 6l4 Am. Lava Corp. 96% Al O 85% 8-9 A-73.7

(underfired) E Alumina AlSiMag 393 Am. Lava Corp. 90% Al O 8-9 fractured 29E Alumina AlSiMag 548 Am Lava Corp. 99.8% M203 85% 6-7 fractured 26E Beryllia BP-96-ll Coors 96% BeO 85% 6-7 fractured a-l Magnesia l87E4 Du-Co Ceramics 89% MgO 85% 4-5 fractured 6-l Magnesia 1e7E77 Du-Co Ceramics 96% M90 85% 43 A 3716 28-5 Magnesium AlSiMag 222 Am. Lava Corp. MgO.SiO2 85% Silicate 27-E Silica 3 Porosity Amersil, Inc. 99% SiO2 85% Fractured 56-T Titania AlSiMag l92 Am. Lava Corp. TiO2 85% 4-5 Fractured (underfired) Z-l Zirconia l72H20 Du-Co Ceramics 95% ZrO2 85% 8-9 A-54.0

44T Alumina AlSiMag Am. Lava Corp. MgO.5iO2 85% 5-6 A65.5

C60 Alumina AP-99C-l2 Coors 99% Al O3 85% 146 Alumina AP-99C-Ll Coors 99% M 0 85% A-66 .4

filled with a reaction product of the ceramic and the additive, if any, with the acid phosphoric, probably a complex metal phosphate.

W her e the ceramic material, such as aluminum oxide is impregnated with a high concentration of phosphoric acid having dissolved therein aluminum phosphate crystals until saturated at from 250 400 F. and is then heat treated, a matefiaTis' produced whichcannot be polished to more than a dull finish, is quite porous and makes an excellent polishing and sharpening stone. This characteristic is also produced where the treatment with phosphoric acid is carried out with dilute acid solutions. It is believed that less reaction product is available to fill the pores, providing a more open and abrasive surface. Here again, the addition of another oxide during treatment substantially increases the hardness of the final product. The starting porous aluminum oxide grades have ranged from about 25 percent to about 60 percent effective porosity and, when subjected to a starved or dilute phosphoric acid treatment, remain quite porous which may account for the excellent polishing and sharpening characteristics of the thus treated material.

The heat treating of the phosphoric acid impregnated ceramic should be initiated at about 150 F. to 350 F. for a short period of time to drive out excess moisture and then the temperature is raised in steps for a series of time intervals until the final cure is accomplished at at least 500 600 F. and preferably at at least 850 900 F. The ceramic will become quite hard at 500 600 F but good electrical resistivity is not achieved until the ceramic is subjected to a temperature of 850 F. or higher. Temperatures above l,000 F. and as high as 3,000 F. have been used with good success. It is found that, once the heat treatment has been carried to above 850 F., the temperature may be increased to well above the normal vitrifying temperatures (e.g., 3,000F.) without producing any shrinkage or change in the original physical dimensions. Further, the high temperatures do not appear to affect the hardness of the material from that of the material heated to 850 F.

While the mechanism of applicants process is not completely understood, it is believed that aluminum phosphate may be formed and deposited in the crystal lattice structure of the aluminum oxide as well as within the voids of the porous ceramic. Further, the phosphates of the impurities and/or additives may be formed and possibly as part of the lattice structure.

As pointed out above, the ceramic materials which are chemically treated and hardened according to one embodiment of the present process display the unique characteristic of exhibiting a low coefficient of friction when sliding against themselves. The coefficient of friction between identical pieces of the material is considerably less than when used in contact with any dissimilar ceramic or metal tested to date.

Although these materials may be operated dry where they are lightly loaded for limited periods of time, the starting friction is considerably higher than when a lubricating material is present. Lubrication may be by a number of different liquids such as tap water, sea water, kerosene, trichlorethylene, lubricating oils, silicone fluids and liquid metals. Dry lubricants such as molybdenum di-sulfide, graphite, wax and the like are also.

suitable. It is possible also to form the lubricant in situ within the pore structure of the bearing.

The bearings can be easily and economically fabricated in a wide variety of shapes and sizes. The untreated ceramic material in the form of partially fired bars or plates is machined to size and shape using conventional high speed steel or carbide tooling. The machined pieces are then chemically treated and hardened at temperatures substantially below normal vitrification temperatures. The hardening occurs with substantially no change in dimensions, thus avoiding expensive diamond machining of the finished part.

The ceramic bearing being fairly porous may be used as the lubricant reservoir analogous to that of sintered bronze bearings. In other instances, the bearing can be operated partially or totally submerged in the lubricant or the non-rotating member can be connected to an external lubricant reservoir.

Typical bearings fabricated of ceramic according to the present invention can withstand repeated water quenching from at least 1,000 F as well as prolonged exposure to temperatures as high as 2,000 F. and as low as 300 F. The compressive strength is on the order of about 125,000 psi or better, and the hardness on the Mohs scale is between 9-l0 or on the order of aboutA-t30-A-90 on the Rockwell scale.

The ceramic materials of Table l were subjected to several slightly different treatments according to this invention, which are: (l) impregnation in phosphoric acid alone; (2) one or more oxide impregnations followed by a single phosphoric acid treatment; or, (3) one or more oxide impregnations alone.

A typical phosphoric acid impregnation process according to the present invention comprises heating the ceramic piece to about 300 600 F. for about 20 minutes, the piece is then immersed in an percent phosphoric acid solution while hot for about 40 minutes. The piece is then placed in an oven and progressively heated from 150 F. to about l,000 F. over a period of about minutes. The piece is then cooled to room temperature.

A typical combination salt and phosphoric acid impregnation process comprises heating the ceramic piece to about 250 450 F. for about 20 minutes. The heated piece is then immersed in the salt solution for about 40 minutes. The piece is removed from the salt solution and cured progressively from F. to about l,000 F. over a period of 120 minutes. The previous steps can be repeated if desired. The piece is then cooled to about 600 F. and immersed in an 85 percent phosphoric acid solution for about 40 minutes. The piece is then placed in an oven and cured over a temperature range of from 150 to about 1,000 F. over a period of about l20 minutes and subsequently cooled to ambient temperature in about 15 minutes.

Fully hardened samples were prepared according to the above treatments from the materials of Table I.

As previously stated, impurities existing in the base material appear to have an effect on the resultant hardness of the treated piece. Therefore, it was decided to artifically add refractory oxides to the porous base structure prior to treating with the phosphoric acid. This was accomplished by impregnating the refractory base material with a nitrate, chloride, acetate or other highly water soluble salt or an acid of the oxide desired, and then converting to the metal oxide by heating slowly to an elevated temperature. Following the oxide impregnation cycle (which may consist of one or more salt or acid treatments), and curing the body was then treated with phosphoric acid.

Tables III, IV, V and VI show the effect of added 0xides to Coors alumina products AP94-L1IAP-85- Ll, AP99L3, and AHP-99, respectively. In these tests, three impregnations of the saturated salt were used (to assure ample loading with the desired oxide), followed by the 85 percent phosphoric acid treatment.

It is interesting to note that these tables show a wide variation in hardness depending on the oxide treatment. In some cases, the hardness is considerably increased over that of the same base material treated with acid only, while in others, the increase is not so marked. The hardness that is obtained with the phosphoric acid treatment only (no oxide impregnation) is listed for comparison purposes.

The Cr O treatment is of special interest in that, when used with the 99 percent, 94 percent and 85 percent Al O base structures, the resulting ceramic is exceptionally high in hardness as compared to all other oxide impregnations tested. The Crr O may be added as a solution of a soluble salt or preferably as a concentrated solution of chromic acid. These four tables also show that the AI-IP-99 material (99 percent A1 is the poorest choice for the base structure of these four types. However, since the AP-99-L3 is also a 99 percent alumina composition, it must be assumed that the hardness is not a factor of the refractory purity alone, but that other factors such as difference is effective pore size is probably responsible for some or all of the noted differences.

Tables VII VIII and IX show the same type of data using aluminum oxides secured from the American Lava Corporation as their types 614 (underfired), 393 and 548. There are 96 percent, 90 percent, and 99.8 percent A1 0 compositions, respectively.

Hardness measurements obtained with Coors 96 percent beryllium oxide for four different salt impregnations is shown in Table X. It is interesting that this base material produces results about equal to the best alumina material tested (Coors AP-99), indicating the refractory skeletal structures other than alumina are definite candidates for the ceramic fabrication method.

Tables XI and XII show hardness results for oxide impregnated magnesia material. While the hardness values are quite low as compared to the alumina or the beryllia, this is to be expected since magnesia, even in its fully fired state, is not a particularly hard material (Mohs 5%).

Table XV lists results obtained with a partially sintered, zirconia refractory base material. This particular underfired zirconia was fabricated from a calcia stabilized but coarse grain material. It is anticipated that a fine grained zirconia, and possible a magnesium oxide stabilized type, would provide better results. Nevertheless, the zirconia also reacts to the chemical hardening method in the same general manner as does the alu mina, magnesia and beryllia and, to a lesser extent, the magnesia silicate and silica materials. Table XVA lists results obtained with aluminum oxide material and Table XVB lists results obtained with titanium dioxide material.

With regard to the effect of pore size, it would be noted that the AHP-99 Coors material has quite large pores, compared to the other Coors material, being on the order of less than 1 micron compared with 2 to 3 microns for the AHP-99 materials. It would appear that the pore size would preferably be less than 2 microns and substantially uniform in size.

HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING COORS AP-94-ll ALUMINA REFRACTORY BASE MATERIAL (Acid Treoted Hardness Mohs 8-9, Rockwell 70.7)

Oxide Salt s n l j Fo rm ed lr 'eregnotion l A|2O3 3)2 7-1 BeO BeCl 5 CO0 Co(NO 3 CdO Cd(NO C-l c Ce(NO3)2 9 CO0 Co(NO 7-3 CuO Cu(NO 7 Fe O FeCl 7-5 0 limo 7-7 U20 LiC H O2 H i 5 EE 5 5 13 MO Ni(NO 0-1 SnO SnClz ll Solt H3PO4 Mobs Rockwell mg. lmpregnotion Hardness Hardness Crooks Remarks 3 9-10 A-7l .5 None 3x 85% 9-l0 A-74.4 None 3x 85% 8-9 A-55 None 3x 85% 8-9 A-63 None 3x 85% 9-l0 A-7l .1 None 3x 85% 8-9 A-74 .8 None 3x 85% 9-10 A-8l .5 None 3x 85% 9-l0 A-bl .0 None 3x 85% 8-9 A-72.5 None 3x 85% 8-9 A-53 .5 Yes 3x 85% 8-9 A -48 .2 Yes 351 V 85% 9-10 hioctured Yes 3x 85% 9-l0 A-73.8 None 3x 85% 9-10 A-75.6 None 3x 85% 9-l0 A-7l .7 None Sample ll Sample 1/ Sarrple Oxide Formed SrO Tl'lO ZnO Oxide Formed Cr O MgO

Oxide Formed Oxide Formed BeO BeO

CaO

CdO

TABLE III (CONTINUED) Salt Salt H PO Mohs Rockwell lmpregnation lmpreg Impregnation Hardness Hardness Cracks Remarks Sr(NO )2 3x 85% 8-9 Fractured Yes Th(NO3) 3x 85% 9-10 A-73 .5 None Ti (C O 3x 85% 9-10 A-73 .5 None H SIW O 0 3x 85% 9-10 A-72 .1 None ZnCl 3x 85% 8-9 A-73 .8 None ZrOCl 3x 85% 9-10 A-76 .1 None (1)FeC| 3x 85% 9-10 A-77 None (1)CrO E REEN HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING COORS AP-85-II ALUMINA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 8-9, Rockwell A-65.9)

Salt Salt H PO Mohs Rockwell lmpregnation lmpreg lmpregnatian Hardness Hardness Cracks Remarks AI(NO 3x 85% 8-9 A-7l None Ce(NO3)2 3x 85% 9-10 A-74 Yes CrO 3x 85% 9-10 A-Bl Nona M (C H O 3x 85% 8-9 A- Yes Shattered During Rockwell Test Ti(C2O 3x 85% 8-9 A-68 Yes Shattered During Rockwell Test I ZrOC| 3x 85% 9-10 A-72 None o efiotw w AM oo c d A o o s s MM HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING COORS AP-99-L3 ALUMINA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 8-9, RockwelFA-70.5)

Salt Salt H PO Mohs Rockwell lrrpregnation lrrpreg. Impregnation Hardness Hardness Cracks Remarks Ce(NO3)2 3X 85% 8-9 A-69.l Yes Exploded in Oven CrO3 3x 85% 9-10 A-80.5 None MgCrO 3X 85% 9-10 A-7l .0 None ZrOCl2 3x 85% 9-10 A-60.l None TABLE VI Salt Salt H3PO Mohs Rockwell lnpregnatlon lrrpreg lmpregnation Hardness Hardness Cracks Remarks Al(NO3I2 3x 85% 8-9 A-60.0 None BeCl2 3x 85% 8-9 A-57 .0 None Be(NO3)2 3x 85% 6-7 A-67.9 None Ca(NO3)2 3x 85% 6-7 Fractured None Cd(NO3)2 3x 85% 4-5 A-55 .0 None USING ALSIMAG 393 ALUMINA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 8-9, Rockwell A- 90% M203) Oxide Salt Salt H PO Mohs Rockwell Sa Ie ll Formed Irrpregnation lrrpreg. lmpregnation Hardness Hardness Cracks Remarks A-4 Cr2O CrO3 3x 85% P 9-10 A-77.0 None A-5 MgCr2O MgCrO4 3x 85% P 9-lO Shattered None A-6 ZrO2 ZrOCIQ 3x 85% P 8-9 A-68.5 None T A ;LE IX HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING ALSIMAG 548 ALUMI NA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 6-7, Rockwell A-N, M 99.8% AI2O Oxide Salt Salt H PO Mohs Rockwell S Ie Formed ln'pregnatlon lmpreg lmpregnation Hardness Hardness Cracks Remarks A-l Cr2O3 CrO3 3x 85% 8-9 Fractured None A-2 MgCr2O MgCrO4 3x 85% 8-9 Fractured None A-3 ZrO ZrOCI2 3x 85% 8-9 A-76.4 None -----T;B]:l( s V I A7 4 HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING COCRS BP-96-Il BERYLLIA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 6-7, Rockwell A-N.M.l

Oxide so" Salt H PO Mohs Rockwell Sam le Farmed Impregnation lmpreg. lmpregnation Hardness Hardness Cracks Remarks B-l Al2O Al(NO 3x 85% 8-9 A-74 None 8-2 Cr2O CrO3 3x 85% 9-l0 A-8l None Shattered in Rockwell Testing 8-4 MgCrO4 MgCrO 3x 85% 9-l0 A-7l None B-3 ZrOZ ZrOCl2 3x 85% 9-10 A-75 None Shattered in Rockwell Testing TABLE Q HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING DU-CO 89% MAGNESIA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 4-5, Rockwell Fractured) Oxide Salt Salt H PO Mohs Rockwell Safile Formed ln'pregnatlon lmpreg. lmpregnatlon Hardness Hardness Cracks Remarks 9-4 Al203 AI(NO3)2 3x 85% 4-5 Fractured None 9-2 Cr O CrO3 3x 85% 8-9 Fractured None 9-3 MgCr2O MgCrO 3x 85% 8-9 A-5l .5 None 9-6 TiO2 Ti2(C2O4) 3x 85% N.M. N .M. M90 Base Disintegrated 9-5 ZrO2 ZrOUB 3X 85% N .M. N .M. M90 Base Disintegrated A i TABLE xu HARDNESS MEASUREMENTS FOR VARIOUS OX-IDE IMPREGNATIONS USING DU-CO 96% MAGNESIA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 4-5, Rockwell A-37.0)

Oxide Salt ll Salt H PO Mohs Rockwell Sarrple Formed lmpregnation Impreg. ln'pregnation Hardness Hardness Cracks Remarks 6-4 Al O Al(NO 3x 85% 3-4 Fractured None 6-2 Cr O CrO 3x 85% 6-7 Fractured None 6-3 MgC r MgCrO4 3x 6-7 A-44 .25 None 6-6 TIO2 Ti (C2O4)3 3x N .M. N .M. Dissolved 6-5 ZrOZ ZrOCI2 3x 85% N .M. N .M Dissolved Hi5 xu e HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING ALSIMAG 222 MAGNESIUM-SILICATE REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 2-3 Rockwell A-N. M.)

Oxide Salt Salt H P0 Mohs Rockwell Sanple Formed impregnation Irrpreg. ln'pregnation Hardness Hardness Cracks Remarks MS-I AI O Al(NO3) 3x 85% 3-4 Fractured None MS-2 Cr O CtO3 3x 85% 8-9 Fractured None MS-3 MgCr O4 MgCrO4 3x 85% 7-8 A-4I None Shattered During Rockwell Test MS -4 ZrO ZrOCl 3x 85% 1-2 Fractured None TABLE l\ HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING AMERSIL '3 POROSITY SILICA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs N. M; Rockwell A-N. M.)

Oxide Salt Salt H3PO4 Mohs Rockwell Sam Ie II Formed Ilrpregnation lnpreg. lmpregnation Hardness Hardness Cracks Remarks S-Z AI O3 Al(NO )2 3x 85% 4-5 Fractured None 5-6 CeO2 Ce(NO 3x 85% 4-5 Fractured None 5-1 0203 CrO 3x 85% 6-7 A-54 .0 None S-3 MgO Mg(C2H3O2) 3x 7 85% 4-5 Fractured None S-5 MgCrO MgCrO 3x 85% 6-7 Fractured None 5-4 ZrO2 ZrOCl 3x 85% 4-5 Fractured None HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING DU-CO ZIRCONIA REFRACTORY BASE MATERIAL (Acid Treated Hardness Mohs 8-9, Rockwell A-54.0)

Oxide Salt Salt H3PO4 Mohs Rockwell Sarrple Formed lrrpregnation Irrpreg. Impregnation Hardness Hardness Cracks Remarks Z-4 Al203 AI(NO3)2 3x 85% 6-7 A-46 .8 None 2-2 0 0 CF03 3x 85% 9-10 A-66.2 None Z-7 MgO M9(C2H3O )2 3x 85% 6-7 Fractured None 2-3 MgCr O MgCrO4 3x 85% 9-I 0 A-58 .0 None Z-S TllOZ Tl'1(NO 3X 85% 6-7 A-55 .3 None Z-6 ZnO Zn(NO 3x 85% 6-7 A-44.7 None 2-5 ZrOz ZrOCI 3x 85% 8-9 A-60.3 None TABLE XVX m n M m S W" HARDNESS MEASUREMENTS FOR VARIOUS OXIDE IMPREGNATIONS USING ALSIMAG 614 96% AI O REFRACTORY BASE MATERIAL PARTIALLY SINTERED AT 2000 F. (Acid Treated Hardness Mohs 8-9, Rockwell A-73l) Oxide Salt t Salt H PO Mohs Rockwell M m impregnation Impreg impregnation Hardness Hardness Cracks Remarks 40T Cr2O CrO 3X 85% 9-I0 A-82.5 None 4IT ZrO ZrOCI2 3x 85% 9-10 A-74.5 None 42r M 6 0 M 00 I 3x 85% 9-10 A-67 .5 None 43T NiO Ni(NO3) 3x 85% 9-10 A-69 .5 None 44T None 85% 5-6 A-65 .5 None TABLE XVB HARDNESS MEASUREMENTS FOR VARIOUS OXlDE lMPREGNATlONS USING ALSIMAG 192 TlTANlA 98% HO; REFRACTORY BASE MATERIAL PARTIALLY SINTERED AT 2000 F (Acid Treated Hardness Mohs 4-5, Rockwell A-N. M.)

Oxide Salt Salt H3PO Mohs Rockwell Sample 9 Formed lmpregnation lmpreg lmpregnarion Hardness Hardness Cracks .Remarks 50-T Cr O CrO3 3x 85% 8-9 A-77.5 None 51 -T ZrO ZrOCl 3x 85% 8-9 A- .0 None 52-1 BeO B e( NO3) 3x 85% 6-7 A-69 .0 None ss-r M 0 Mg(C2H O 3x 85% 6-7 Fractured 54-1 A1 0 A|(NO 3x 85% -6 Fractured 55-T M 0 0 M900 3x 85% 9-10 A-65 .0

56-T None 85% 4-5 Fractured Tables XVIXXIX show the hardness of selected oxide impregnations using Coors AP-99-L3 alumina base materials which have been treated with multiple salt impregnations to illustrate the effect on hardness of varying the amount of added oxide prior to the final acid treatment. In the preceding tables, all samples were impregnated with the salt solution three times. The following impregnations were varied from as few as one time to a maximum of eleven times, and included a final phosphoric acid treament. The base materials and oxide impregnations tested in this manner were selected from the materials of Table I.

Table XVI shows the effect of 1 through 11 chromic base material, while Table XVIA shows the effect of one through eight chromic oxide impregnations with AP-94-ll alumina base material and Table XVII shows one through five impregnations with AP-94-l2 material. These tables show the definite increase in hardness with increase in number of oxide impregnations. The rate of increase in hardness is also seen to decrease as the number of impregnations increase. This would appear to follow since there is probably less and less interstital space for the oxides with each successive treatment. Specific gravity and porosity tests bear this out.

TABLE xvi Oxide Salt Salt H3PO Mohs Rockwell SLJe Fried lrrpregnation lnpreg. lmpregnation Hardness Hardness Cracks Remarks -L Cr O CrO 1x 9-10 A73.2 None 81 -L Cr2O CrO3 3x 85% 9-10 A-80.4 None 82-L G 0 CrO 5x 85% 9-10 A-83 .9 None 83-L Cr O CrO 7x 85% 9-10 A87.6

84-L Cr2O CrO3 9x 85% 9-10 A-88.3 None 85-L Cr O CrO 11x 85% 9-10 A-88.9 None I h I W W V TABLE XVIA WW-Mmflw HARDNESS VARlATlON WITH NUMBER OF CHROMlC OXIDE IMPREGNATIONS USING COORS AP-94-ll ALUMINA REFRACTORY BASE MATERIAL Oxide Salt ll Salt H3PO Mohs Rockwell m med lmpregnation lnpreg. lrrpregnation Hardness Hardness Cracks Remarks L-B Cr203 CrO3 1x 85% 9-10 A-76.4 None L-9 Cr2O3 CrO3 2x 85% 9-10 A-80.7 None 3X Cr2O CrO3 3x 85% 9-10 A-Bl .8 None 0 0 CrO3 4x 85% 9-10 5X G20 CrO3 5x 85% 9-10 A-85 .0 None 6X Cr2O CrO3 6x 85% 9-10 A-85 .0 None 7X Cr2O CrO3 7x 85% 9-10 A-86.0 None ex 0 0 00;, 8x 85% 9-10 A-87.0 None Oxide Salt I Salt H PO Mohs Rockwell M m lmpregnation Irrpreg. Impregnation Hardness Hardness Cracks Remarks L-7 0 O03 Ix 85% 9-l0 I A-75.8

K-8 Cr2O 00:; 2x 85% 9-l0 A-79 .6

L-4 Cr2O CrO3 3x 85% 9-l0 A-8l .5

L-5 Cr203 CrO3 4x 85% 9-10 A-83 .9

2-5 020 0'03 5x 85% 9-l0 A-86 .0

3-5 Cr2O CrO3 7x 85% 9-l0 A-83 .0

4-5 0 0 CrO 9x 85% 9-l0 A-84 .0

5-5 Cr O3 CrO3 I Ix 85% 9-10 A-85 .0

These tables show that there is very little difference in the hardness results obtained between the AP-94-ll and the AP-94-l2 materials. The difference between these two base materials is in their sintering temperatures, respectively 1,700F. and 2,l30F.

Table XVIII shows the results obtained with chromic oxide impregnations on Coors AHP-99 alumina material. While the hardness increases with the number of chromic oxide impregnations, the hardness numbers obtained for a given number of treatments is much less than those obtained with chromic oxide treatment of Coors AP-99-L3 material of Table XVI. Since these alumina materials are both99 percent aluminum oxide, and both have the same effective porosity of about 40 percent, the differences measured must be arresult of the different pore size. The AHP-99 material has larger pores believed to be on the order of 2-3 microns average, while the AP-99-L3 average pore size appears to be on the order of 0.6-0.7 microns.

TABLE XVIII HARDNESS VARIATION WITH NUMBER OF CHROMIC OXIDE IMPREGNATIONS USING COOIS AHP-99 ALUMINA REFRACTORY BASE MATERIAL Tables XIX and XX show the effect on hardness for one through five impregnations of zirconium oxide into base materials of AP-94-I2 and AHP-99 alumina, respectively. Again, the AP-94 material produces greater hardness than the AI-IP-99 for comparable impregnations. Also, while the AP-94 material impregnated with zirconium oxide does not produce as hard an end product as does the chromic oxide impregnation, the reverse is true when considering the AHP99 material. Again, the explanation is undoubtedly connected with differences in pore size and/or impurities in the base material.

Tables XXI and XXII show similar tests to those just described, except that the impregnant was a concentrated solution of magnesium chromate instead of a concentrated solution of zirconyl chloride.

Oxide Salt I, Salt H3PO4 Mohs Rockwell M L-: r r nc d lmpregnalion ln'preg. lmpregnation Hardness Hardness Cracks Remarks O-b Cr O CrO Ix L-l 0 0 G03 2x 85% 8-9 A-57.4 None O-7 020 CrO 3x 85% 9-10 A-69 .2 None L-2 Cr O CrO 4x 85% 8-9 A-68.7 None 0-8 Cr O CrO3 5x 85% 9-l0 A-73.0 None 3-U G 0 CrO 7x 85% 9-10 A-80.0 None 4-U Cr O CrO 9x 85% 9-]0 A-76.0 None 5-9.. Q 3 .w. a f' ..-.Q- H

""TABLE XIX HARDNESSVARIATIONS WITH NUMBER OF ZIRCONIUM OXIDE IMPREGNATIONS USING COORS AP-94-l2 ALUMINA REFRACTORY BASE MATERIAL Oxide Salt sell H PO M0115 Rockwell Sample Formed lmpregnation lmpreg. lmpregnal'ion Hardness Hardness Cracks Remarks Y-I ZTOQ ZrOCI2 1x 85% 8-9 A-71.9 None K -6 ZrOZ ZrOC l 2 2x 85% 8-9 A-74 .6 None -T ZrO2 ZIOCIZ 3X 85% 9-10 A-70.0 None b-T ZrO2 ZrOClz 5x 85% 9-10 A-7S.0 None 7-T ZrO ZrOCIz 7x 85% 9-10 A-73.0 None 8-T ZTO ZrOC|2 9x 85% 9-10 A-80.5 None 9-T ZrOQ ZTOCI2 I Ix 85% 9-10 A-78 .0 None TABLE xx HARDNESS VARIATIONS WITH NUMBER OF ZIRCONIUM OXIDE IMPREGNATIONS USING COORS AHP-99 ALUMI NA REFRACTORY BASE MATERIAL Oxide Salt ll Salt H PO M0115 Rockwell Sample m lnpregnation lmpreg. Impregnation Hardness Hardness Cracks Remarks Y-2 ZrO2 ZrCI2 1x 85% 5-6 A-55 .5 None Y-4 ZrO2 ZTCI 2x 85% 9-10 A63.5 None K-5 ZrO2 ZrCI2 3x 85% 9-10 A-61.7 None Y-5 ZrO ZrCl2 4x 85% 9-10 A-71.6 None TABLE XXI Oxide sell Salt H PO Mohs Rockwell San'ple Formed Impregnalion In'preg. lmpregnation Hardness Hardness Cracks Remarks M-1 MgCr2O MgC r04 Ix 85% 9-10 A-66 None M-2 MgCr2O MgCrO 3x 85% 9-10 A-72 None M-3 MgCr2O MgCrO 5x 85% 9-10 A-70 None -1. m mummw TABLE XXII I HARDNESS VARIATION WITH NUMBER OF MAGNESIUM CHROMITE IMPREGNATIONS USING COORS AHP-99 ALUMINA REFRACTORY BASE MATERIAL Oxide Salt Salt H3PO Mohs Rockwell Sanple Formed lrtpregnation Impreg lmpregnation Hardness Hardness Cracks Remarks M-4 MgC r MgCrO 1x 85% 6-7 A None M-5 MgCr2O MgCrO 3x 85% 9-10 A-53 None M-6 MgCr2O MgCrO 5x 85% 9-10 A-l None Tables XXIII and XXIV are for ceric oxide l m niegthe impregnant. Table XXVII is again for the AP-94 i 3 32 6 g zg gi material's, "a p base meteriel but using a IE1 mixture of conaentrated tlve y. a e coverst e materia wit cons centrated cobalt nitrate solution used as the impregt? fll nant. Table XXVI is for the same base material but I" Sg l 9f 9 f whata pge g 59 35 using a concentrated siIico-tungstic acid solution for chromite when cured.

USING COORS AP-94-I2 ALUMINA REFRACTORY BASE MATERIAL Oxidev Salt Salt H PO Mohs Rockwell Sanple Formed In'pregnation Inpreg. lm regnation Hardness Hardness Cracks Remarks 4-A Fe O Cr O (I)FeCI3+ Ix 85% 6-7 A-72 None (I)CrO3 I -A Fe O3Cr2O (l)FeCI 3x 85% 9-10 A-75 None (I)CrO3 2-A Fe2O3Cr2O (I)FeCI3+ 4x 85% 9-IO A-77 None (I)CrO 3-A Fe2O Cr2O3 (I)FeCI3+ 5X 85% 9-10 A-82 None (l)CrO K zTrc o riiabase Instant has been isa"f6rsi's shown as Tables XXVlll and XXIX. These are for a coarse grain, calcia stabilized, 95 percent zirconia underfired refractory material with chromic oxide and zirconium oxide impregnations as shown.

TABLE XXVIII HARDNESS VARIATIONS WITH NUMBER OF CHROMIC OXIDE IMPREGNATIONS USING DU-CO, CALCIA STABILIZED, 95% ZIRCONIA BASE MATERIAL Oxide Salt Salt H PO4 Mohs Rockwell Sar Ie Formed lmpregnation Inpreg Impregnation Hardness Hardness Cracks Remarks 20-2 Cr O C r0 3x 85% 6-7 A -69 .5 None 2I -Z Cr2O CrO 5x 85% 6-7 A -78 .5 None 22-Z Cr2O CrO 7x 85% 6-7 A-77 None 23-Z None None None 85% 8-9 A-54 None TABLE XXIX HARDNESS VARIATIONS WITH NUMBER OF ZIRCONIUM OXIDE IMPREGNATIONS USING DU-CO, CALCIA STABILIZED, 95% ZIRCONIA BASE MATERIAL Oxide Salt Salt H3PO Mohs Rockwell Scale Formed lrrpregnation- Irrpreg lmpregnation Hardness Hardness Cracks Remarks 23-2 ZrO ZrOCl 3x 85% 6-7 A-65 None 24-2 ZrO ZrOCIz 5x 85% 6-7 A-66 None 25-2 ZrOz ZrOClz 7x 85% 6-7 Fractured 26-Z None None None 85% 8-9 A-54 None W A series of multiple phosphoric acid treatments of the Coors Ap-94, AP85 and Al-lP-99 alumina base materials has been investigated. The results are shown in Tables XXX, XXX] and XXXII. For the most part,

has been first impregnated with three chromi cacid applications prior to the final phosphoric acid treatments. Again, one phosphoric acid treatment appears to be optimum.

lew- MULTIPLE ACID IMPREGNATIONS USING COORS AP-94-I2 ALUMINA REFRACTORY BASE MATERIAL Salt Salt H PO Acid Mohs Rockwell Sanple Impregnation lmpreg. Impregnation lmpreg. Hardness Hardness Cracks Remarks P-I None 85% Ix 8-9 A-68.7 None P-2 None 85% 2x 8-9 A-67.8 None P-3 None 85% 3x 6-7 A67.7 None P-4 None 42-l/2% Ix 4-5 A-64.8 Yes Fractured P-5 None 42-I/2% 2x 6-7 A-58 .7 Yes Fractured P-6 None 42-l/2% 3x 6-7 A-58.5 None Fractured 

2. The method of claim 1 wherein at least the surface of the particles are selected from the group of inorganic refractories consisting of oxides of aluminum, barium, beryllium, calcium, cerium, chromium, cobalt, copper, gallium, hafnium, iron, lanthanum, magnesium, manganese, molybdenum, nickel, niobium, silicon, tantalum, thorium, tin, titanium, tungsten, uranium, vanadium, yttrium, zinc, zirconium and mixtures thereof.
 3. The method of claim 1 wherein the solution is chromic acid.
 4. The method of claim 1 wherein the finely divided refractory particles include an amount of reinforcing fibers, which fibers retain their identity even in the fully hardened body.
 5. The method of claim 1 which includes as a last step the impregnation of the hardened refractory body with phosphoric acid and heating and curing said body by raising the temperature of said body to at least about 600*F. but below the vitrification temperature of the refractory particles in increments over a period of time sufficient to completely dry and harden the body.
 6. The method of claim 1 wherein a clay or inorganic chromium compound binder is mixed with the finely divided particles prior to forming the structure which serves to hold the particles together during the impregnation and curing cycles.
 7. A method of producing a chemically hardened refractory body which comprises: closely packing finely divided discrete refractory oxide particles having a vitrification temperature in excess of 600*F. to form a porous body essentially devoid of vitreous bonding between the particles; impregnating said porous body with a solution of an inorganic chromium compound in a solvent selected from the group consisting of water, alcohol and ether, which compound is capable of being converted to an oxide on being heated to a temperature below the vitrification temperature of said refractory oxide particles; drying and curing said impregnated body by heating same to a temperature below the vitrification temperature of the particles but sufficient to convert the chromium compound in situ to the oxide; and, repeating the impregnation and heat curing steps at least once to harden and densify the body.
 8. The method of claim 7 wherein the oxide particles are packed into a mold of predetermined shape at least during the initial impregnation and cure steps.
 9. The method of claim 7 wherein the oxide particles are mixed with a clay or inorganic chromium compound binder to provide a plastic mass which is formed into a predetermined shape prior to the initial impregnation and cure steps.
 10. The method of claim 7 wherein the particles are selected from the group consisting of oxides of aluminum, barium, beryllium, calcium, cerium, chromium, cobalt, copper, gallium, hafnium, iron, lanthanum, magnesium, manganese, molybdenum, nickel, niobium, silicon, tantalum, thorium, tin, titanium, tungsten, uranium, vanadium, yttrium, zinc, zirconium and mixtures thereof.
 11. The method of claim 7 wherein the solution is chromic acid.
 12. The method of claim 7 wherein the finely divided refractory particles include an amount of reinforcing fibers, which fibers retain their identity even in the fully hardened body.
 13. The method of claim 7 which includes as a last step the impregnation of the hardened refractory body with phosphoric acid and heating and curing said body by raising the temperature of said body to at least about 600*F. in increments over a perioD of time sufficient to completely dry and harden the body.
 14. A method of producing a chemically hardened refractory body which comprises: providing a non-vitrified porous skeletal core of inorganic refractory material having a vitrification temperature in excess of 600*F.; impregnating said core with a solution of an inorganic chromium compound in a solvent selected from the group consisting of water, alcohol and ether which compound is capable of being converted to an oxide on being heated to a temperature below the vitrification temperature of said refractory material; drying and curing said impregnated core by heating the core to a temperature below the vitrification temperature of said refractory material but sufficient to convert the chromium compound to the oxide in situ; and, repeating the impregnation and heat curing steps at least once to harden and densify the core.
 15. The method of claim 14 wherein the skeletal core is of a material from the group consisting of oxides of aluminum, barium, beryllium, calcium, cerium, chromium, cobalt, copper, gallium, hafnium, iron, lanthanum, magnesium, manganese, molybdenum, nickel, niobium, silicon, tantalum, thorium, tin, titanium, tungsten, uranium, vanadium, yttrium, zinc, zirconium and mixtures thereof.
 16. The method of claim 14 wherein the solution is chromic acid.
 17. The method of claim 14 which includes as a last step the impregnation of the hardened refractory body with phosphoric acid and heating and curing said body by raising the temperature of said body to a least about 600*F. but below the vitrification temperature of the refractory particles in increments over a period of time sufficient to completely dry and harden the body.
 18. The method of claim 6 wherein the clay binder is selected from the group consisting of bentonite, kaolin and Kentucky ball clays.
 19. The method of claim 6 wherein the binder is chromic acid.
 20. The method of claim 9 wherein the clay binder is selected from the group consisting of bentonite, kaolin and Kentucky ball clays.
 21. The method of claim 7 wherein the binder is chromic acid.
 22. A method according to claim 7 wherein the finely divided oxide particles are pretreated with a solution of a inorganic chromium compound capable of being converted to an oxide on being heated and the particles are heated to convert the treating solution thereon to an oxide before being closely packed to form a porous structure.
 23. A method according to claim 10 wherein the finely divided oxide particles are pretreated with a solution of a inorganic chromium compound capable of being converted to an oxide on being heated and the particles are heated to convert the treating solution thereon to an oxide before being closely packed to form a porous structure.
 24. The method of claim 1 wherein the finely divided particles are metal particles.
 25. The method of claim 1 wherein the finely divided particles are carbide particles.
 26. The method of claim 1 wherein the finely divided particles are complex oxides. 